As well known, in a spread spectrum system, a transmitting-side signal is spread over chips having a smaller interval than a symbol interval, and thus the spread spectrum system is sensitive to a clock timing offset. For example, inter-chip interference caused by a chip delay is a primary cause of degradation of the system. Therefore, it is required to accurately estimate and compensate for chip and symbol clock timing offsets.
In a spread spectrum system, each data symbol is spread by a code having good correlation characteristics with other codes, that is, a code which has a peak value only when the code is multiplied by the same code at a receiving side and has a very small value when the code is multiplied by a chip-delayed code or a code of another pattern. When a sample clock offset occurs during an analog-to-digital converting (ADC) process, the same amount of offset occurs in chip clock timing also, and thus sample positions of respective data symbols having a peak value differ from each other by the offset upon despreading.
As an existing method of estimating a symbol and a chip clock timing offset, an early-late gate algorithm has been widely known. This algorithm always involves comparing a power value at a sample position ahead of an ideal sample point with a power value at a sample position behind the ideal sample point, thus requiring a large amount of computation in a system having a high data rate. Also, in a system in which a timing error fluctuates within a symbol unit, there is a high probability that an ideal sample point is not an accurate timing, and thus an error increases upon downsampling.
As another existing method of estimating a symbol and a chip clock timing offset, the Mueller-Muller algorithm is used. This algorithm requires only one sample per symbol, thus requiring a small amount of computation. However, this algorithm is sensitive to a carrier offset, and its timing estimation performance is degraded in a system with many carrier offsets.
This work was supported by the ICT R&D program of MSIP/IITP, Republic of Korea. [14-911-01-003, Development of software-based measuring equipment for enhancing inspection of radio station]